This invention relates generally to rotary drum vacuum filters of the so-called single cell type in which the entire drum interior is subjected to a uniform vacuum usually applied through the center barrel with or about which the drum rotates. As the drum rotates through slurry in a tank, filtrate is drawn through perforations in the drum surface while solids deposit thereon as a cake. The filtrate is withdrawn from the drum through the center barrel while cake is scraped from the drum surface.
Filters of this type are especially adapted to use as pre-coat filters in which, as is well known, a cake of solids is initially formed to act as the filter medium upon which further cake forms and from which it is scraped by a slowly advancing blade.
More particularly, this invention is directed to an improved filter drum construction providing superior strength and enhanced filtrate removal characteristics.
In some prior filters, the drum interior is divided into sectors by solid plates radiating from the center barrel to connect to the drum wall. Openings in the center barrel apply vacuum and withdraw filtrate. Drums of this type are expensive, moreover since the sectors are separate, excess filtrate in a sector not entirely removed in a given cycle is not diverted to another sector. A typical form of this type drum is disclosed in German Pat. No. 233,316.
Another prior construction employs shallow pans in the drum interior. These are not secured to the drum wall. The vacuum in the drum interior is uniform and excess filtrate spilling from the pans reports to the bottom of the drum. Some designs have stationary pans and may also employ siphons to remove filtrate. Typical filters of this type are disclosed in U.S. Pat. Nos. 898,312 and 1,064,702. A more recent design, disclosed in U.S. Pat. No. 4,083,787, has shallow open topped pans radiating from and rotating with the center barrel to swap filtrate that accumulates in the drum. Vacuum is applied and filtrate withdrawn through the center barrel of the drum. The pans are spaced from the drum wall to avoid interference with filtration.
The primary object of the present invention is the provision of a drum in a rotary drum vacuum filter that is specifically designed as a single cell drum in which the interior of the drum is functionally undivided insofar as vacuum application is concerned.
An important additional object is the provision of a filter of the type described having a plurality of filtrate removal pans that are integral to the drum structure thereby to enhance drum strength.
An additional object is the provision of filtrate removal pans that divert all filtrate away from inside the filtering surface on each pickup sweep thus avoiding liquid pressure on the under side of filter cake.
A further object is the provision in a filter drum of the type described of a filtrate pan and manner of connecting it to the inner drum wall that, in addition to enhancing filtrate pickup, also provides drainage ports to prevent or limit accumulation of residual liquid between the pan and interior drum surface during descent of the drum.
The foregoing and probably other objects of the invention are attained by a filter construction comprising a tank and a drum mounted for rotation therein in which the drum has closed opposite ends and a perforated cylindrical wall. Conduit means which advantageously include the center barrel connect the drum to vacuum. At least one filtrate scoop is mounted in the drum for rotation therewith. The scoop is formed as an elongated pan extending between the closed drum heads or end walls and is defined by the drum ends, a bottom wall, and two spaced apart side walls. The pan has an open top that faces in the direction of intended rotation of the drum; and the leading edge of the outer sidewall of the pan is connected along its entire length to the inner surface of the perforated curved drum wall. A closed conduit provides communication between the bottom of the pan and the center barrel or shaft thence to vacuum.
In the preferred embodiment, a plurality of pans are provided. They are all welded to the drum ends and are an integral part of the drum structure, especially effective in resisting collapse of the drum when internal vacuum is applied.
Securing the leading edge of the pan wall to the drum surface in accordance with the invention not only provides strength to the drum, but also materially enhances filtrate removal. This is so because as the drum rotates through the filtrate then ascends, the pan wall diverts all liquid away from the perforated drum wall into the pan. Filtrate in the pan is discharged through the conduit as the pans rotate. The pans are relatively deep hence confine filtrate and firmly direct it to the withdrawal conduit. If there is excess filtrate it is diverted away from the perforated wall to the bottom of the drum.
The inner surface of the drum is constantly wet because of the continual application of vacuum; and as the drum descends, this filtrate flows downwardly along the drum wall. In accordance with a feature of this invention, accumulation of residual liquid between the inner drum wall and outer pan wall is avoided by the provision of spaced drainage ports in the pan wall adjacent its line of connection to the drum surface. These ports are sized and spaced to quickly drain and thus prevent accumulation of residual liquid during drum descent. However, the ports are small enough that they do not impair filtrate pickup when the pans rotate through submergence durng the filtrate scooping and ascending travel of the drum.
In order that the invention may be more readily understood and carried into effect, reference is made to the accompanying drawings and the description thereof which are offered by way of example only and not in limitation of the invention the scope of which is defined only by the appended claims and equivalents embraced thereby rather than any preceding description.